1. Technological Field
This invention is generally concerned with the field of opto-electronic systems and devices. More specifically, embodiments of the present invention relate to a header assembly for use in various optoelectronic devices.
2. Related Technology
Transistor headers, or transistor outlines (“TO”), are widely used in the field of opto-electronics, and may be employed in a variety of applications. As an example, transistor headers are sometimes used to protect sensitive electrical devices, and to electrically connect such devices to components such as printed circuit boards (“PCB”).
With respect to their construction, transistor headers often consist of a cylindrical metallic base with a number of conductive leads extending completely through, and generally perpendicular to, the base. A glass hermetic seal between the conductive leads and the base provides mechanical and environmental protection for the components contained in the TO package, and electrically isolates the conductive leads from the metallic material of the base. Typically, one of the conductive leads is a ground lead that may be electrically connected directly to the base.
Various types of devices are mounted on one side of the base of the header and connected to the leads. Generally, a cap is used to enclose the side of the base where such devices are mounted, so as to form a chamber that helps prevent contamination or damage to those device(s). The specific characteristics of the cap and header generally relate to the application and the particular device being mounted on the base of the header. By way of example, in applications where an optical device is required to be mounted on the header, the cap is at least partially transparent so to allow an optical signal generated by the optical device to be transmitted from the TO package.
Although transistor headers have proven useful, typical configurations nevertheless pose a variety of unresolved problems. Some of such problems relate specifically to the physical configuration and disposition of the conductive leads in the header base. As an example, various factors conspire to compromise the ability to precisely control the electrical impedance of the glass/metal feedthru, that is, the physical bond between the conductive lead and the header base material. One such factor is the fact that there is a relatively limited number of available choices with respect to the diameter of the conductive leads that are to be employed. Further, the range of dielectric values of the sealing glass typically employed in these configurations is relatively small. And, with respect to the disposition of the conductive leads, it has proven relatively difficult in some instances to control the position of the lead with respect to the through hole in the header base.
Yet other problems in the field concern those complex electrical and electronic devices that require many isolated electrical connections in order to function properly. Typically, attributes such as the size and shape of such devices and their subcomponents are sharply constrained by various form factors, other dimensional requirements, and space limitations within the device. Consistent with such form factors, dimensional requirements, and space limitations, the diameter of a typical header is relatively small and, correspondingly, the number of leads that can be disposed in the base of the header, sometimes referred to as the input/output (“I/O”) density, is relatively small as well.
Thus, while the diameter of the header base, and thus the I/O density, may be increased to the extent necessary to ensure conformance with the electrical connection requirements of the associated device, the increase in base diameter is sharply limited, if not foreclosed completely, by the form factors, dimensional requirements, and space limitations associated with the device wherein the transistor header is to be employed.
A related problem with many transistor headers concerns the implications that a relatively small number of conductive leads has with respect to the overall performance of the device wherein the transistor header is used. Specifically, devices such as semiconductor lasers operate more efficiently if their driving impedance is balanced with the impedance at the terminals. Impedance matching is often accomplished through the use of additional electrical components such as resistors, capacitors and transmission lines such as microstrips or striplines. However, such components cannot be employed unless a sufficient number of conductive leads are available in the transistor header. Thus, the limited number of conductive leads present in typical transistor headers has a direct negative effect on the performance of the semiconductor laser or other device.
In connection with the foregoing, another aspect of many transistor headers that forecloses the use of, for example, components required for impedance matching, is the relatively limited physical space available on standard headers. In particular, the relatively small amount of space on the base of the header imposes a practical limit on the number of components that may be mounted there. In order to overcome that limit, some or all of any additional components desired to be used must instead be mounted on the printed circuit board, some distance away from the laser or other device contained within the transistor header. Such arrangements are not without their shortcomings however, as the performance of active devices in the transistor header, such as lasers and integrated circuits, depends to some extent on the physical proximity of related electrical and electronic components.
The problems associated with various typical transistor headers are not, however, limited solely to geometric considerations and limitations. Yet other problems relate to the heat generated by components within, and external to, the transistor header. Specifically, transistor headers and their associated subcomponents may generate significant heat during operation. It is generally necessary to reliably and efficiently remove such heat in order to optimize performance and extend the useful life of the device.
However, transistor headers are often composed primarily of materials, Kovar® for example, that are not particularly good thermal conductors. Such poor thermal conductivity does little to alleviate heat buildup problems in the transistor header components and may, in fact, exacerbate such problems. Various cooling techniques and devices have been employed in an effort to address this problem, but with only limited success.
By way of example, solid state heat exchangers may be used to remove some heat from transistor header components. However, the effectiveness of such heat exchangers is typically compromised by the fact that, due to variables such as their configuration and/or physical location relative to the primary component(s) to be cooled, such heat exchangers frequently experience a passive heat load that is imposed by secondary components or transistor header structures not generally intended to be cooled by the heat exchanger. The imposition on the heat exchanger of such passive heat loads thus decreases the amount of heat the heat exchanger can effectively remove from the primary component that is desired to be cooled, thereby compromising the performance of the primary component.
As suggested above, the physical location of the heat exchanger or other cooling device has various implications with respect to the performance of the components employed present in the transistor header. On particular problem that arises in the context of thermoelectric cooler (“TEC”) type heat exchangers relates to the fact that TECs have hot and cold junctions. The cold junction, in particular, can cause condensation if the TEC is located in a sufficiently humid environment. Such condensation may materially impair the operation of components in the transistor header, and elsewhere.
Another concern with respect to heat exchangers is that the dimensions of typical transistor headers are, as noted earlier, constrained by various factors. Thus, while the passive heat load placed on a heat exchanger could be at least partly offset through the use of a relatively larger heat exchanger, the diametric and other constraints imposed on transistor headers by form factor requirements and other considerations place practical limits on the maximum size of the heat exchanger.
Finally, even if a relatively large heat exchanger could be employed in an attempt to offset the effects of passive heat loads, large heat exchangers present problems in cases where the heat exchanger, such as a TEC, is used to modify the performance of transistor header components such as lasers. For example, by virtue of their relatively large size, such heat exchangers are not well suited to implementing the rapid changes in laser performance that are required in many applications because such large heat exchangers heat up and cool down relatively slowly. Moreover, the performance of the laser or other component may be further compromised if the heat exchanger is located relatively far away from the laser because the rate at which heat can be transferred with respect to the laser or other component is at least partially a function of the distance between the component and the heat exchanger.
In view of the foregoing discussion, what is needed is a transistor header having features directed to addressing the foregoing exemplary concerns, as well as other concerns not specifically enumerated herein. An exemplary transistor header should implement a relatively high I/O density without increasing the relative diameter of the header. Moreover, the exemplary transistor header should be configured to precisely control the electrical impedance and permit location of various components in relatively close proximity to the active components, such as a laser, within the header without violating applicable form factors or other geometric and dimensional standards. Finally, the exemplary transistor header should include features directed to facilitating a relative improvement in heat management capability within the transistor header.